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Share-Based Payments (Tables)
 12 Months Ended
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Summary of stock option activity and related information
A summary of the Company’s stock option activity and related information follows:
 
Shares
 
Weighted-Average
Exercise Price
 
Remaining
Contractual Life
(in years)
Aggregate
Intrinsic
Value
(in thousands)
Outstanding at December 31, 2015
1,803,574

 
$
6.28

 
7.78
$
1,540

Granted
1,747,495

 
1.69

 
 
 
Exercised
(656,532
)
 
0.41

 
 
 
Cancelled or forfeited
(870,069
)
 
5.86

 
 
 
Outstanding at December 31, 2016
2,024,468

 
$
4.41

 
8.73
$
841

Exercisable at December 31, 2016
1,139,514

 
$
5.09

 
8.08
$
779

Vested and expected to vest at December 31, 2016 (1)
1,879,868

 
$
4.49

 
8.66
$
834

(1)Represents the number of vested options, plus the number of unvested options expected to vest.
Summary of status and changes of unvested restricted stock
A summary of the status of unvested restricted stock as of December 31, 2016 and 2015, and changes during the year ended December 31, 2016 are presented below:
 
Restricted
Stock
 
Weighted-Average
Grant Date
Fair Value
Unvested at December 31, 2015
41,657

 
$
11.05

Vested
(19,507
)
 
10.62

Unvested at December 31, 2016
22,150

 
$
11.43
Summary of restricted stock unit activity
A summary of the status of restricted stock units is presented below:
 
Restricted
Stock Units
 
Weighted-Average
Grant Date
Fair Value
Unvested at December 31, 2015
150,932

 
$
2.85

Vested
(134,499
)
 
2.83

Cancelled
(13,100
)
 
2.76

Unvested at December 31, 2016
3,333

 
$
4.09
Estimation of fair value of stock option granted using Black-Scholes option-pricing model assumptions
The fair value of each non-employee stock option is valued on grant date and revalued as it vests using the Black-Scholes option-pricing model based on assumptions noted in the following table:
 
Year Ended December 31,
 
2016
 
2015
 
2014
Risk-free interest rate
1.08-2.38%
 
1.19-2.26%
 
1.67-2.04%
Expected dividend yield
—%
 
—%
 
—%
Expected option life (years)
10
 
10
 
10
Expected stock price volatility
69.92-92.09%
 
67.24-92.40%
 
57.65-80.98%
Employees and Directors Stock Options  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Estimation of fair value of stock option granted using Black-Scholes option-pricing model assumptions
The fair value of each stock option granted to employees and directors was estimated on the date of grant using the Black-Scholes option-pricing model based on the assumptions noted in the following table:
 
Year Ended December 31,
 
2016
 
2015
 
2014
Risk-free interest rate
1.23-2.38%
 
1.42-1.92%
 
1.67-2.02%
Expected dividend yield
—%
 
—%
 
—%
Expected term (in years)
5.5-6
 
5.75-6
 
5.75-6
Expected volatility
71.44-73.42%
 
69.06-74.11%
 
60.00-69.58%